1. Field of the Invention
The present invention relates to a synchronized communication system in which all communication devices connected to a transmission path are synchronized with a reference time in the communication system.
2. Description of Related Art
In manufacturing systems, generally, a single control device such as a programmable controller or a personal computer periodically exchanges command data and response data with a plurality of devices such as sensors, relays, and servo drives to perform control. Each device is connected to a transmission path, and the exchange of command data and response data is performed in constant communication cycles through communication. In particular, in motion control systems for machine tools, chip mounters, and the like, a motion controller forms a control loop together with servo drives through a transmission path.
Therefore, a synchronized communication system in which the exchange of command data and response data can be realized in steady communication cycles is demanded to allow these devices to operate in synchronization with the communication cycles. In order to achieve such enhancement in performance of motion control, it is necessary to synchronize the timings of starting the processing of command data received by the servo drives and the like from the motion controller so that the amount of synchronization deviation thereof can be minimized and it is also necessary to shorten as much as possible the communication cycle in which the motion controller and the servo drives exchange command data and response data.
In the related art, in order to synchronize all communication devices connected to a transmission path with a reference time in a communication system, a method has been adopted in which a communication device that manages the reference time informs one or more other communication devices of the reference time.
In industrial fields such as on a production line, devices are often connected in series to a transmission path in order to save wiring. In a case where communication devices are connected in series to a transmission path, the transfer delay time of a relay communication device affects performance.
For example, it is assumed that a first communication device manages a reference time and a plurality of second communication devices operate in synchronization with the first communication device. In a manufacturing system, the first communication device corresponds to, for example, a programmable controller or a personal computer, and the second communication devices correspond to, for example, servo drives, sensors, relays, or the like.
In order to allow a plurality of second communication devices or devices connected to the second communication devices to operate synchronously, a reference time held by each second communication device must have been obtained by correcting the reference time held by the first communication device using the transmission path delay time.
For example, Japanese Unexamined Patent Application Publication No. 10-142361 discloses an example in which a communication device that manages a reference time determines a return delay time (the time required for data to reciprocate) with each communication device connected thereto, measures a delay time to each communication device on the assumption that the time required for outgoing communication and the time required for incoming communication are equal, and corrects the clock time of each communication device so that the clock time can be synchronized with the reference time.